The present invention relates generally to the controlled formation and/or orientation of large molecules, such as nanotubes, on surfaces, and more particularly to formation of carbon nanotubes on surfaces for making nanoscopic-scale electronic devices such as memory arrays, configurable logic and other computer elements.
During the past several decades there has been a nearly constant exponential growth in the capabilities of silicon-based microelectronics leading, for example, to tremendous advances in our computational capabilities. Silicon-based microelectronics, however, can be made only so small. That is, there is a size limitation smaller than which silicon-based microelectronics cannot be fabricated. Specifically, the National Technology Roadmap for Semiconductors (SEMATECH, Austin, 1997) suggest that silicon-based microelectronics, which are typically said to follow xe2x80x9cMoore""s Lawxe2x80x9d, will continue only to about the year 2010. At this time, two factors are expected to bring Moore""s scaling to an end. First, fundamental physical limitations will be reached for both device elements and wire interconnects that will prevent current designs from functioning reliably. Second, the concurrent exponential increase in fabrication (FAB) facility cost is expected to make it uneconomical to consider increasing integration levels further (using silicon technology) even if it is physically meaningful.
These factors, and the expected benefits that could be derived from further dramatic increases in computational power in the relatively near future, have led many to consider new devices and computer architectures. In particular, there has been considerable interest in developing the concept of molecular electronics. Molecular-based electronics can in principle overcome the fundamental physical and economic limitations of silicon-based microelectronics; it is physically possible to have single molecular devices. For example, a conformational change that varies the conjugation in a molecule could behave as a switch or rectifier.
Investigation has taken place into manipulation of molecules at surfaces for electronic applications. Liu, et al, in xe2x80x9cControlled Deposition of Individual Single-Walled Carbon Nanotubes on Chemically Functionalized Templates,xe2x80x9d Chem. Phys. Lett. 303 (1999)125-129 report procedures for producing individual, short carbon nanotube segments and for their deposition on chemically functionalized nanolithographic templates. Specifically, a patterned self-assembled monolayer is formed on a surface and a carbon nanotube is adsorbed onto the surface in an orientation corresponding to the pattern. The authors also describe connection of an individual carbon nanotube between two electrodes.
Monolayers have been used to provide molecular electronic devices. Collier et al. describes the use of a Langmuir-Blodgett film of rotaxane molecules interposed between lithographically fabricated wires of micron-scale diameter (Science Vol. 285, p. 391, 1999). This system is useful for read-only memory devices, however, as the configurable elements involve irreversible oxidation of the rotaxane.
Other studies report a single carbon nanotube constructed as an electronic switch (Collins, et al., Science 278 (1997)100), and a room-temperature transistor (Tans, et al., Nature 393 (1998) 49).
To date, there has been considerable progress in characterization of the electrical behavior of individual or small numbers of molecule devices. However, a significant need exists for improvement in molecule-scale electronics, especially for integrating bistable and switchable devices for high-density memory arrays.
The present invention provides a series of nanoscopic-scale electronic elements, methods of making nanoscopic-scale electronic elements, and methods of use of nanoscopic-scale electronic elements.
In one aspect, the invention provides a nanoscopic-scale electronic device. The device is defined by an electrical crossbar array that includes at least one nanoscopic wire. The crossbar array can be of a variety of configurations such as a 1xc3x978 array, 8xc3x978 array, etc. The array can include contact electrodes in electrical contact with various wires, for example, by covalent attachment. Crossbar arrays provided according to the invention have densities up to about 1012/cm2.
In another aspect the invention provides techniques for making nanoscopic-scale electronic devices. In one embodiment, the invention involves forming a nanoscopic wire on a surface in a pattern dictated by a chemically patterned surface. The nanoscopic wire can be a pre-formed wire, in which case the method involves applying the pre-formed wire to the surface in the pattern. Alternatively, the nanoscopic wire can be grown on the surface in the pattern. The chemically patterned surface can be patterned to direct assembly or growth of the nanoscopic wire in a predetermined orientation useful for a particular electronic device.
In another embodiment the invention provides a method involving growing a nanoscopic wire in the presence of an electric field. The field is of intensity sufficient to orient the growth of the wire. This method can, optionally, be used in combination with a method involving growing a nanoscopic wire on a self-assembled monolayer. In all methods, nanoscopic wire growth can be carried out via chemical vapor deposition (CVD).
In other embodiments, the invention provides a method involving forming a nanoscopic wire on a surface in a pattern dictated by a mechanically patterned surface or by gas flow.
In another aspect, the invention provides methods of using electronic devices. In one aspect, a method of the invention involves providing a crossbar array comprising at least two wires in crossbar array orientation, where the wires are free of contact with each other, and bringing the wires into contact with each other. The wires are contacted at a crossbar array junction at which they are alternately brought into contact with each other and released from contact with each other. In one embodiment the wires are nanoscopic wires.
Another aspect of the present invention provides an article comprising a self-assembled monolayer defining a delineated pattern. At least two crossed wires are associated with the self-assembled monolayer in which at least one of the wires is a nanoscopic wire. In another aspect, the invention provides an article comprising an electric crossbar array comprising at least two crossed wires defining a memory element able to be switched between at least two readable states. The device is free of auxiliary circuitry other than the at least two crossed wires defining the memory element.
In another aspect, the present invention provides a method comprising switching a memory element of a crossbar array between xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states by alternatively biasing, at similar and opposite polarity, wires that cross the array to define the element.
In another aspect, the present invention provides an article comprising an electric crossbar array comprising at least two crossed nanoscopic wires defining a memory element capable of being switched reversibly between at least two readable states.
In another aspect, the present invention provides an article comprising an electrical crossbar array comprising at least two crossed nanoscopic wires defining a memory element capable of being switched between at least two readable states. The memory element is non-volatile.
In another aspect, the present invention provides an article comprising an electrical crossbar array comprising at least two crossed wires defining a diode. The device is free of auxiliary circuitry other than the at least two crossed wires defining the diode.
In another aspect, the present invention provides a method comprising providing a mixture of metallic nanotubes in semiconducting nanotubes. The method also involves separating the metallic nanotubes from the semiconducting nanotubes.
In all of the embodiments of the invention, preferred nanoscopic wires are nanotubes.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.